1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a “semiconductor device” refers to a device that can function by utilizing semiconductor characteristics; a transistor, an electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique by which a transistor is formed using a thin semiconductor film formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as integrated circuits (ICs) or image display devices (display devices). A silicon-based semiconductor material is widely known as a material for a thin semiconductor film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor including an amorphous oxide semiconductor film containing indium (In), gallium (Ga), and zinc (Zn) as an active layer of a transistor is disclosed (see Patent Document 1).
Further, a transistor including an oxide semiconductor film in which a portion being in contact with a gate insulating film is in an amorphous state and a portion being in contact with a source electrode or a drain electrode is in a crystal state is disclosed (see Patent Documents 2 and 3).
Furthermore, a transistor including a stack of oxide semiconductor films with different compositions which are used for a channel formation region in order to improve field-effect mobility and decrease off-state current is disclosed (see Non-patent Document 1).